1. Field of Invention
The invention relates to a zoom lens used in photographic deices, such as digital cameras, and, in particular, to a zoom lens with both merits of compact size and high resolution.
2. Related Art
As the manufacturing techniques of CCDs (Charge-Coupled Device) with more than two million pixels are becoming better, a zoom lens with higher resolution for the digital camera is then demanded. One also wants to lower the production cost for zoom lens and to shorten the length of zoom length. However, the previous patents referring to zoom lens proposed for the digital camera, such as the U.S. Pat. No. 4,701,034 and U.S. Pat. No. 4,859,042, show numbers of lenses, heavy weight, and long length.
The invention provides a zoom lens that has a more compact size and satisfies the requirement of high resolution.
The zoom lens disclosed herein is designed into a frame of four lens groups and aspherical lenses are used in lens groups. Starting from the side near objects, the first, the second, the third, and the fourth lens groups are the ones with positive, negative, positive, and positive refractive power, separately. When the zoom lens varies from the short focal length to the long one, i.e., wide angle to telephoto lens, the first lens group is fixed, the second lens group is moved from the object plane (closest to the object on the lens) to the image plane, the third and the fourth lens groups are independently moved from the image plane to the object plane to compensate for the image plane shift of the second lens group. Therefore, a clear image can be formed on the image plane even when the zoom lens focuses on objects at different distances. Each of the third and the fourth lens groups contains at least one aspherical lens. When the zoom lens varies its focal length from a shorter one (wide angle) to a longer one (telephoto lens), the distortion is less than 2%. This satisfies the optical requirement of high-resolution zoom lens for a digital camera.